ANTIBODY OR IMMUNOGLOBULIN - CLASSES OF ANTIBODIES OR IMMUNOGLOBULINS

            An antibody, also known as an immunoglobulin (Ig), is a large, Y-shaped protein produced mainly by plasma cells that is used by the immune system to identify and neutralize pathogens such as bacteria and viruses.
             Antibodies are secreted by B cells of the adaptive immune system, mostly by differentiated B cells called plasma cells. Antibodies can occur in two physical forms, a soluble form that is secreted from the cell to be free in the blood plasma, and a membrane-bound form that is attached to the surface of a B cell and is referred to as the B-cell receptor (BCR). The BCR is found only on the surface of B cells and facilitates the activation of these cells and their subsequent differentiation into either antibody factories called plasma cells or memory B cells that will survive in the body and remember that same antigen so the B cells can respond faster upon future exposure.

            Antibodies are glycol-proteins belonging to the immunoglobulin superfamily. They constitute most of the gamma globulin fraction of the blood proteins. They are typically made of basic structural units –each with two large heavy chains and two small light chains. There are several different types of antibody heavy chains that define the five different types of crystallisable fragments (Fc) that may be attached to the antigen-binding fragments. The five different types of Fc regions allow antibodies to be grouped into five isotypes.

            CLASSES OF IMMUNOGLOBULIN (ISOTYPES)

            Human sera contain different types of immunoglobulins such as – IgG, IgA, IgM, IgD and IgE in the descending order of concentration. Ig stands for immunoglobulin and G, A, M, D and E stand for γ (gamma), α (alpha), μ (mu), δ (delta) and ε (epsilon), respectively.

            IgG:- This is the major serum immunoglobulin constituting of about 80% of the total. It is a 7 S molecule and has a molecular weight of 150,000. IgG may occasionally exist in a polymerized form. It is distributed approximately equally between the intravascular and extravascular compartments. It contains less carbohydrate than other immunoglobulins. It has a half life of approximately 23 days. The normal serum concentration of IgG is about 8-16 mg per ml.

            IgA:- IgA is the second most abundant class constituting about 10-13% of the serum immunoglobulin. The normal serum level is 0.6-4.2 mg per ml. It has a half life of 6-8 days. Serum IgA is principally a monomeric 7 S molecule of molecular weight 160,000. It is the major immunoglobulin in the colostrum, saliva and tears.

            IgM:- IgM constitutes 5-8% of serum immunoglobulin, with a normal level of 0.5-2 mg per ml. It has a half life of about 5 days. It has a molecular weight of 9000,000 to 10,000,000, hence called as the millionaire molecule. Most of IgM is intravascular in distribution. Phylogenetically, IgM is the oldest immunoglobulin class. It is also the earliest immunoglobulin synthesized by the foetus, beginning by about 20 weeks of age.

            IgD:- IgD resembles IgG structurally. It is present in a concentration of about 3 mg per 100 ml of serum and is mostly intravascular. It has a half life of about 3 days. IgD and IgM occur on the surface of un-stimulated B lymphocytes and serves as recognition receptors for antigens. Combination of cell membrane bound IgD or IgM with the corresponding antigen leads to specific stimulation of B cell – either, activation and cloning to produce antibody antibody or suppression.

            IgE:- It is a serum immunoglobulin of 80 S molecule containing a molecular weight 190,000, with a half life of about 2 days. It resembles IgG structurally. Normal serum contains only traces (a few nanograms per ml), but greatly elevated levels are seen in atopic (type 1 allergic) conditions such as asthma, hay fever and eczema. Children living in insanitary conditions, with a high load of intestinal parasites, have high serum levels of IgE. IgE is chiefly produced in linings of the respiratory and intestinal tracts.

            The antibody recognizes a unique molecule of the harmful agents, called an antigen, via the Fab’s variable region. Each tip of the "Y" of an antibody contains a paratope (analogous to a lock) that is specific for one particular epitope (similarly analogous to a key) on an antigen, allowing these two structures to bind together with precision. Using this binding mechanism, an antibody can tag a microbe or an infected cell for attack by other parts of the immune system, or can neutralize its target directly (for example, by blocking a part of a microbe that is essential for its invasion and survival). Depending on the antigen, the binding may impede the biological process causing the disease or may activate macrophages to destroy the foreign substance.

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